drm/mgag200: switch to drm_*_get(), drm_*_put() helpers

Use drm_*_get() and drm_*_put() helpers instead of drm_*_reference()
and drm_*_unreference() helpers.

drm_*_reference() and drm_*_unreference() functions are just
compatibility alias for drm_*_get() and drm_*_put() and should not be
used by new code. So convert all users of compatibility functions to
use the new APIs.

Generated by: scripts/coccinelle/api/drm-get-put.cocci

Signed-off-by: Cihangir Akturk <cakturk@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1502454794-28558-15-git-send-email-cakturk@gmail.com